Anatomical Terms: (Refer FIG. 1, FIG. 2 and FIG. 3)
Atlas (C1):
It is the most superior (first) cervical vertebra of the spine which supports the globe of the head. The Atlas' chief peculiarity is that it has no body, it is ring-like, and consists of an anterior and a posterior arch and two lateral masses.
Axis (C2):
It is the second cervical vertebra of the spine and is named the axis or epistropheus. It forms the pivot upon which the first cervical vertebra (C1—the atlas), which carries the head, rotates. The most distinctive characteristic of this bone is the strong odontoid process (“dens”) which rises perpendicularly from the upper surface of the body. That peculiar feature gives to the vertebra a rarely used third name: vertebra dentata.
The atlas along with the Axis forms the joint connecting the skull and spine. The atlas and axis are specialized to allow a greater range of motion than normal vertebrae. They are responsible for the nodding and rotation movements of the head.
The Atlas and Axis are important neurologically as it houses the cervico-medullary junction that is packed with vital neural structures.
Dens or Odontoid Process:
It exhibits a slight constriction or neck where it joins the body. The dens acts as a pivot that allows the atlas and attached head to rotate on the axis, side to side.
Occipital Bone:
It is a saucer-shaped membrane bone situated at the back and lower part of the cranium, is trapezoidal in shape and curved on itself. It is pierced by a large oval aperture, the foramen magnum, through which the cranial cavity communicates with the vertebral canal.
Atlanto-Occipital Joint:
It is the articulation between the atlas and the occipital bone and consists of a pair of condyloid joints. The atlanto-occipital joint is a synovial joint which allows the head to nod up and down on the vertebral column.
Atlanto-Axial Joint:
It is a joint in the upper part of the neck between the first and second cervical vertebrae; the atlas and axis. The atlanto-axial joint is of a complicated nature. It consists of no fewer than four distinct joints. It provides a significant range of movements (majority of the rotational movement of neck). There is a pivot articulation between the odontoid process of the axis and the ring formed by the anterior arch and the transverse ligament of the atlas. There are two lateral atlanto-axial joints that are relatively flat, unlike the other vertebral articulations thereby, providing the rotational movement between C1 with head and C2, apart from providing support. Besides this, there is minimal translational, vertical and lateral bending movements that can occur between C1-2. Vertical movement of C2 within C1 occurs with rotation of C1 over C2. This is known as coupling. With coupling, dens is at its summit with C1 and C2 in neutral position and it is at its lowest point with C1 rotated over C2 to either side. The coupling movement makes the C1-2 joint extremely efficient.
About Atlanto Axial Dislocation (AAD):
The translational movement between C1-2 is restricted by the C1 anterior arch (anteriorly) and the transverse ligament (posteriorly). The ligaments and the joint capsule of lateral atlanto-axial joints, prevents excessive rotation and lateral displacement. The orientation of facets of C1-2 lateral masses prevents vertical dislocation. Atlanto axial dislocation is a condition in which, there is abnormal displacement between C1 and C2 in single or more than one plane. This dislocation is in short labeled as AAD. Such dislocation may be reducible (correctable on neck movement in a direction opposite to the displacement) or irreducible. A major risk in the case of AAD is spinal cord compression, which can cause severe neurological injuries.
Causes for AAD:
The causes vary from congenital (abnormal orientation of the joints seen in individuals born with deformed joints), ligamentous laxity due to infection or its squeale, inflammatory diseases like rheumatoid arthritis or trauma leading to fracture of the restricting odontoid.
Symptoms of AAD:
Pain in the neck area, especially in the back of head and neck. Movement is painful and restricted. Spinal cord compression is a major risk in this event. The odontoid process may compress the spinal cord, causing severe neurological injuries and even death. Neurological damage may cause change in walking ability, weakness in the arms and legs, loss of bladder or bowel control and difficulty in breathing. Displacement and neck movements must be avoided at all costs.
Treatment for AAD:
Strict immobilisation of the neck is of paramount importance. Treatment includes symptomatic measures and cervical immobilisation in reduced position, usually beginning with a rigid cervical collar. Close monitoring of the patient is essential due to the risk of displacement of the neck bones, potentially causing spinal cord compression and neurological damage. Pain killers are advised for the pain. Surgery may be needed to stabilize the spine. (Ref: http://www.mdhil.com/atlanto-axial-dislocation-symtoms-and-treatent/)
As discussed above, AAD may cause neural compromise. Therefore sometimes surgical intervention becomes a necessity.
A neurosurgeon has following options for treating AAD:
1. Reduction/removal of neural compression followed by C1-2 fusion
2. Direct reduction of C1-2 joints and fusion of C1-2 lateral joints                C1-2 fusion with cable fixation (after reduction, either by maneuvering the neck position in reducible AAD or cervical traction or by joint distraction in the irreducible variety)        C1-2 transarticular screw fixation        C1-2 lateral mass/isthmus fixation        
3. Odontoid screw fixation (cases of acute trauma with non-comminuted fracture of dens)
4. Implanting an artificial atlanto-dental joint
Limitations of Above Methods:
Fusing the C1-2 Lateral Joints:
Most of the techniques are directed at fusing the C1-2 lateral joints in reduced position that avoids further dislocation. However, such fusion leads to gross restricting of the neck movements. Such restricted movement hampers a person's daily activity like driving, playing outdoor games that require rotational movement of the head.
Artificial Atlanto-Dental Joint:
Although this is physiological and provides stability without compromising the neck movements. However, the ONLY possible way to fix such central joint is through the oral cavity that is potentially infected and therefore is not desirable. Furthermore, use of such joint is not feasible in congenital anomalies that have oblique C1-2 lateral joints or in inflammatory diseases like rheumatoid arthritis where the vertical settling is of common occurrence along with dislocation. Additionally, it cannot provide the lateral or translational movements.
Proposed Solution by the Inventor:
The inventor being a neurosurgeon himself, has designed and developed an implant which could reduce if not eliminate totally, the drawbacks of presently available implants or fusion techniques and the patient can have better mobility of the neck and therefore better quality of life. This implant is in the form of an artificial joint or a dynamic stabilizer for the lateral atlas-axis (C1-2) joints.
Advantages of the Present Invention:
1. Stability of C1-2 without Compromising the Neck Movements:
The artificial lateral C1-2 joints would provide good stability without compromising the normal movements of neck i.e., axial rotation close to normal, small degree of lateral tilt and minimal translational movement in antero-posterior and lateral direction (multiple degrees of freedom)
2. Avoiding the Infected Route:
The placement of artificial C1-2 lateral joints of the present invention is through the back of neck, thereby avoiding the infected cavity.
3. Use in Congenital Anomalies:
The approach requires opening of lateral C1-2 joints and drilling the facetal surfaces flat and close to normal. The inventor remodels a deformed joint as close to a normal joint as possible. Following the drilling, it is feasible to use the implant in most of the cases of congenital AAD.
4. Use in Inflammatory Arthritis
Settling is often seen in cases of AAD due to rheumatoid arthritis. The new implant is likely to increase the height apart from providing dynamic stabilization
5. Providing the Naturally Occurring Lateral and Translational Movement with Coupling to Make it the Most Efficient.
The implant has been designed to provide coupling and minimal translational movement. This allows more degrees of freedom of movement and makes it efficient.
6. Universal Design
The implant can be fixed in any individual with AAD irrespective of the angles and orientation of C1-2 joints                As is clear from the above, instead of modifying the techniques of lateral C1-2 joint fusion or focusing on the placement of artificial atlanto-dental joint through a potentially infected cavity, inventor came up with very practical solution of providing stability of C1-2 joints without restricting the neck movements. Furthermore, it does not use the infected route and can be used in patients with congenital anomalies or inflammatory arthritis.        
Prior Art and its Drawbacks:
Various doctors and researchers have been working to solve the problem as cited above. The approaches used are discussed in the patents below:
PatentSapplicationDisadvantage of prior artComparison with presentNono.inventioninvention1.WO2012145971The ONLY possible way to fixThe present invention is fixedsuch central joint is through thethrough the back of the neck andoral cavity that is potentially not through oral cavity therebyinfected, reducing the post-operativeCannot be used in deformed C1-2 infection.joints (congenital AAD and Can be used in Congenital andBasilar invagination) or inflammatory AADinflammatory AAD with settling.Does provide the naturallyDoes not provide the naturallyoccurring translational andoccurring translational and coupling movements morecoupling movement reducing the degrees of freedom).axial freedom of movement 2.WO2013/177314A1This implant is bulky in nature.The present invention requiresIt is difficult to be used in opening of joints, scraping of thecongenital Irreducible Atlanto- cartilage and even drilling it flat inaxial dislocation as it reduction case of oblique shape (congenitalwould require opening and drilling variety or traumatic locked facets)of joints. Furthermore the tendencyto accommodate the implant. Thisis to get dislocated in antero- helps in reduction of joints. Theposterior direction due to implant is not bulky and placedobliquity, producing a constant through posterior approachdrag on the implant.thereby avoiding the potentiallyIt provides minimal rotational infected cavity.movement and is likely to be Also the artificial joint replacesrestricted further due to soft tissue the natural joint making it moreentanglement.physiological, avoids the softNormal C1-2 lateral joints are a tissue and provides greater rangepre-requisite for this implant.of movement.The placement of this device isthrough standard posteriorapproach to cervical spine.3.CN 1669539 AAgain it uses the potentiallyThe route is surgically clean. Also,infected trans-oral route for though an intact dens provides aplacement. The hook with sheet good stability. However, even inreplaces the transverse ligament its absence or destruction orand is connected to the C1 lateral hypoplasia, the rail road wouldmass on one side. The hook allow action only in rotationalrequires intact dens and of course plane and is stable. Also, it can beanatomically normal C1-2 joints used in oblique lateral C1-2 joints.4.CN1973785AThe ONLY possible way to fixThe present invention is fixedsuch central joint is through the through the back of the neck andoral cavity that is potentially not through oral cavity therebyinfected. reducing the post-operativeOnly rotational movement is infection. Multiple degrees ofpossible freedom of movement5.CN2390561YThe ONLY possible way to fix The present invention is fixedsuch central joint is through the through the back of the neck andoral cavity that is potentially not through oral cavity therebyinfected. Only rotational reducing the post-operativemovement is possible infection. Multiple degrees offreedom of movement6.CN103142331AIt is a fusion device. Can provideUse of present invention allowsstability but at the cost of mobilitythe patient to have better mobilityof the neck.7.US7566346Use in sub-axial cervical spine.The intended design is to provideProvides rotation and translationalall degrees of freedom ofmovement both in lateral and movement as provided by theantero-posterior direction (threenatural C1-2 lateral joints. Thedegrees of motion). This cannot beunique rail and channelused in C1-2 lateral joints as thearchitecture on the periphery withrotational motion required is insome play and the centralaround the odontoid that acts as agyroscopic design makes it stablepivot and not around the jointwith significant freedom ofitself. Even if the channel is mademovement with leading to antero-circular for the ball, the structure posterior, lateral and verticalof artificial joint makes it risky fordislocation.lateral and vertical dislocation.8.US20070123863This provides the spacer effect,The present design providesproviding vertical distraction and maximum movement in rotationalwith preservation of someplane and with naturally occurringmovement translational, angular lateral or angular movement alongand rotational movement. But the with coupling.degree of movement availablecannot be utilized in C1-2 lateralmasses as it is minimal. The excesslateral and angular movementprovided by the artificial facets isdangerous at C1-2 level